1 Field of the Invention
The present invention relates to semiconductor laser amplifiers which can be used as a repeater or an amplifying distributor on-line switch in a time-division multiplexing optical fiber communication system. Other applications include bar code scanners and a super speed video camera which can take billions of pictures per second with a shutter speed of approximately 10.sup.-11 second.
2. Description of Prior Art
Presently there are two types of semiconductor laser light amplifiers: (i) the traveling wave amplifier (TWA) and (ii) the resonant type or Fabry Perot amplifier (FPA). In both types, direct currents are used to inject minority carriers into the active channel for lasing action. The essential difference between the two types is the reflectivities at the ends of the active channel.
In a TWA, the reflectivities are made as small as possible. Considerable amount of research work is still being performed with respect to reducing reflectivities. The input light wave is amplified as it travels from one end of the active channel to the other end. The reflected waves are also amplified. If the product of the reflectivities and the round trip gain exceeds 1, the "amplifier" would emit light without any light input creating a useless amplifier. If the product is a significant fraction of 1, an interference pattern would develop between the main wave and the reflected waves. The interference pattern is "noise" and is a severe limiting factor on the gain of a TWA.
In an FPA, the reflectivities are usually on the order of a few tenths. Thus, light photons are partially trapped between the two reflecting ends, and their number multiplies due to stimulated emission as the photons travel back and forth in the active channel. The amplification factor increases exponentially with the densities of the minority carriers in the active channel. At a certain density level, the product of the reflectivities and the round trip amplification factor is equal to 1, and the "certain density level" is then referred to in the technical literature as the critical density which is denoted here as N.sub.c. It should be noted that the operating minority carrier density level in a TWA can be considerably higher than the critical density of an FPA made with the same material because the reflectivities of the TWA are much lower.
Operating at its critical density, the total loss of photons in an FPA, including photon losses at its two ends, is substantially equal to the number of newly generated photons due to stimulated emission. If the device is left alone without further light input, the number of photons travelling back and forth in its active channel would remain constant, with a constant output at the output facet. The device would not be useful as a light amplifier because it would provide an output without any input.
However, if an FPA is operated at a carrier density level N slightly below critical, (N&lt;N.sub.c, N.sub.c -N&lt;&lt;1), and a light signal is applied to its input, the photon density in the active channel would build up gradually, until the net photon loss is equal to the number of input photons. The following would happen as N approaches N.sub.c :
(i) the amplifier gain increases; PA1 (ii) the buildup time, or response time increases; and PA1 (iii) the amplifier bandwidth narrows. PA1 (i) vanishingly small reflectivities at the two end facets; and PA1 (ii) an operating minority carrier density less than the critical density but approaching the critical density.
There is a limit as to how close N can be made to approach N.sub.c. Because each stimulated emission means the loss of one minority carrier, with the same injection dc current, N reduces as the light input increases. The effect of gain reduction with input light intensity is called "gain compression" and is common to all laser light amplifiers. However, the gain reduction is far more severe in an FPA because its gain depends on 1/(N.sub.c -N) rather than on N itself.
The present invention provides a higher performance than both the TWA and the FPA because it does not require the following limiting conditions of operation:
In addition, the signal sampling effect which is characteristic of the present invention is important to the applications mentioned in paragraph 1, Field of the Invention.